Well, I would say, that if the board is working properly, but the only factor you give it is time, and then you are logged out, that's because of security reasons. My servers are programmed to time a cookie out, so it doesn't stay indefinitely. If you login from a public terminal, walk away for a day, week, or month, and others use the computer, that cookie will stay live, and they would have access to your account. So I programmed it not to do that.
Now, I do allow the option to stay logged in, or be logged in on each visit. So, my server will program the cookie for your computer, and keep you logged in. That's what I do, because no one else uses my computer.
If you are using that feature, and are still logged out after a day or so, my guess is you have software, like a Norton, Mcaffee, AVG, or another type of security program or firewall, that expires all cookies automatically for their own "security" reasons, which is, of course, out of my control.
I hope that explains a bit more. Cookie problems are a tough thing to diagnose, because each computer is different, each browsers security is set different, and each security / anti-virus / firewall is different.
Let me know if the problem, other than outlined/explained above, persists.